1. Field of the Invention
The present invention relates to a liquid crystal display, and more particularly, to a color-correction method and apparatus for a liquid crystal display. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for effectively correcting a color balance.
2. Discussion of the Related Art
Generally, a liquid crystal display (LCD) controls light transmittance of each liquid crystal cell in accordance with video signals, thereby displaying a picture. An active matrix LCD including a switching device for each liquid crystal cell is suitable for displaying a dynamic image. The active matrix LCD uses a thin film transistor (TFT) as a switching device.
However, the LCD has a disadvantage in a response time due to inherent characteristics of the liquid crystal such as viscosity and elasticity, etc.
Referring to FIG. 1, upon implementation of a moving picture, a conventional LCD cannot express desired color and brightness because one frame fails to achieve the target brightness when data are changed from one level to another level due to its slow response time. Accordingly, a motion-blurring phenomenon appears in the moving picture, and a display quality is deteriorated due to a reduction in a contrast ratio and hence a visual recognition by a user becomes poor.
In order to overcome such a slow response time in the LCD, U.S. Pat. No. 5,495,265 and PCT International Publication No. WO99/05567 have suggested a scheme for driving a liquid crystal display at a high speed using a look-up table for modulating a voltage of input data. This high-speed driving scheme modulates input data, as shown in FIG. 2.
Referring to FIG. 2, a conventional high-speed driving scheme modulates input data VD and applies the modulated data MVD to the liquid crystal cell, thereby obtaining desired brightness MBL. Accordingly, an LCD employing such a high-speed driving scheme reduces a motion-blurring phenomenon in a moving picture, thereby displaying a picture with desired color and brightness.
This high-speed driving scheme compares current input data with previous data to modulate the input data using look-up table information, as shown in Table 1.
TABLE 13 V4 V5 V6 V7 V8 V3 V6.6 V9.3 V11.8 V 13.7 V15.4 V4 V2.2 V6.8 V9.1 V11.2 V12.9 V5 V2.0 V3.2 V7.3 V 9.3 V11.1 V6 V1.65 V 2.6 V4.0 V 8.0 V 9.8 V7 V1.6 V2.6 V3.5 V4.9 V 8.8 V8 V1.6 V2.4 V3.1 V4.4 V 6.2 V
In the above table, the furthermost left column is for a data voltage VDn−1 of the previous frame Fn−1 while the uppermost row is for a data voltage VDn of the current frame Fn.
According to Table 1, the look-up table information suggested in the conventional high-speed driving scheme modulates input data VD on the basis of a data voltage relationship between the previous frame Fn−1 and the following current frame Fn. The data voltage relationship is expressed by the following equations:VDn<VDn−1→MVDn<VDn  (1)VDn=VDn−1→MVDn=VDn  (2)VDn>VDn−1→MVDn>VDn  (3)
In the above equations, VDn−1 represents a data voltage of the previous frame, VDn is a data voltage of the current frame, and MVDn represents a modulated data voltage.
As shown in Table 1 and equation (1), the conventional high-speed driving method is to compare the data voltage VDn−1 of the previous frame Fn−1 with the data voltage VDn of the current frame Fn. If the data voltage VDn inputted at the current frame Fn is smaller than the data voltage VDn−1 of the previous frame Fn−1 as a result of such a comparison, it is modulated to be smaller.
Further, from Table 1 and equations (2) and (3), the conventional high-speed driving method applies the input data voltage to the liquid crystal cell without a data modulation when the data voltage VDn inputted at the current frame Fn is equal to the data voltage VDn−1 of the previous frame Fn−1. On the other hand, the input data voltage is modulated to be greater when the data voltage VDn inputted at the current frame Fn is larger than the data voltage VDn−1 of the previous frame Fn−1.
However, the conventional high-speed driving method has a problem in that a color expression may be further distorted upon displaying colors.
A single dot includes sub-cells for expressing three primary colors of light, that is, red (R), green (G), and blue (B) colors. A color is determined by the sum of red, green, and blue lights emitted from the sub-cells.
If data are continuously changed between the previous frame Fn−1 and the current frame Fn as shown in a moving picture, a desired color cannot be expressed when sub-cells having a data value to be changed between frames and sub-cells having a data value to be unchanged between frames co-exist in one dot.
Referring to FIG. 3, red data VRD are modulated to be greater than an input data value at the previous frame Fn−1. They are not modulated when a data value of the current frame Fn becomes equal to that of the previous frame Fn−1. Green data VGD are modulated to be greater than the input data value at both the previous frame Fn−1 and the current frame Fn. On the other hand, blue data VBD are modulated to be greater than the input data value at the previous frame Fn−1 and modulated to be smaller than the previous frame Fn−1 at the current frame Fn. As mentioned above, unmodulated red data VRD are applied as input data to the liquid crystal cell, whereas the green data VGD and the blue data VBD are modulated and then applied to the liquid crystal cell.
As shown in FIG. 4, brightness BLG and BLB of the green sub-cell and the blue sub-cell appear to have a brightness level lower than a desired brightness level indicated by the oblique line portions at the current frame Fn due to a slow response characteristic of the liquid crystal. Therefore, the picture has contrast lower than intended colors to display. On the other hand, brightness BLR of the red sub-cell maintains brightness of the previous frame Fn−1 at the current frame Fn. As a result, the conventional high-speed driving scheme may distort a color balance upon displaying colors due to a defective data modulation method.